How to convert a PDF document to an older version

Acrobat 9 Pro can be used to re-distill PDF 1.7 into any "lower" version of PDF. Look at the Distiller settings... Up to you to decide if it's "not too expensive".

Depending on the exact feature subset of PDF-1.7 used in your files, even (Free) Ghostscript may be able to do a good quality transformation to PDF-1.2, PDF-1.3 or PDF-1.4 file format version.

This is the Ghostscript command line to create a PDF-1.4:

 gs                        \
  -sDEVICE=pdfwrite        \
  -dCompatibilityLevel=1.4 \
  -o output.pdf            \
     input.pdf 

Note:
The more recent versions of Ghostscript can abbreviate the parameter -sOutputFile=... to -o .... It also implicitly also sets -dBATCH -dNOPAUSE. So it is much shorter to type and much easier to avoid typing errors.

If you're using Ubuntu 11.04 (or probably any Gnome distro) and can view the PDF in the built-in Document Viewer, you can then Print to file to save it as a PDF-1.5. (Save A Copy, in contrast, just creates a byte-identical copy of the file.) I did this today to convert a PDF from 1.6 to 1.5, so now I can use it with the FPDF lib.

In addition to the Ghostscript solution suggested above, you can use the "Reduce File Size" option in the "Document" menu of Adobe Acrobat 8 or 9, and choose the compatibility level you desire.

Additionally, Acrobat 8 or Acrobat 9 Pro can run the PDF Optimizer in Batch Processing to convert multiple files at once; simply create a new sequence with no commands. In the sequence's Output Settings dialog, check the "PDF Optimizer" box and click the Settings button - enable or disable any optimization settings you'd like, and set the "Make compatible with" dropdown near the top to the appropriate Acrobat/PDF version you desire. Run the batch sequence on the files/folders you wish to convert, and PRESTO, hundreds of documents converted in minutes!

I used the PDF Optimizer/Batch Processing method to convert 297 PDF files in about 3-4 minutes (most were 1-page flyer type documents). It's also worth noting that the PDF Optimizer can't convert secured documents ;)

Adobe Acrobat DC is $15/mo and its "Optimizing PDFs" dialog can do what you need

Look under the "Make compatible with" dropdown